Optical reflectivity experiments have been conducted on single crystals of the Kondo insulator YbB12 in order to obtain its optical conductivity, σ(ω). Upon cooling below 70 K, a strong supression of σ(ω) is seen in the far-infrared region, indicating the opening of an energy gap of ∼ 25 meV. This gap development is coincident with a rapid decrease in the magnetic susceptibility, which shows that the gap opening has significant influence on magnetic properties. A narrow, asymmetric peak is observed at ∼ 40 meV in σ(ω), which is attributed to optical transitions between the Yb 4fderived states across the gap. In addition, a broad peak is observed at ∼ 0.25 eV. This peak is attributed to transitions between Yb 4f -derived states and p-d band, and is reminiscent of similar peaks previously observed for rare-earth hexaborides.

Infrared optical conductivity [σ(ω)] of the intermediate valence compound YbAl3 has been measured at temperatures 8 K ≤ T ≤ 690 K to study its microscopic electronic structures. Despite the highly metallic characters of YbAl3, σ(ω) exhibits a clear pseudogap (strong depletion of spectral weight) of about 60 meV below 40 K. It also shows a strong mid-infrared peak centered at ∼ 0.25 eV. Energy-dependent effective mass and scattering rate of the carriers obtained from the data indicate the formation of a heavy-mass Fermi liquid state. These characteristic results are discussed in terms of the hybridization states between the Yb 4f and the conduction electrons. It is argued, in particular, that the pseudogap and the mid-infrared peak result from the indirect and the direct gaps, respectively, within the hybridization state.

Optical conductivity data of the intermetallic compounds (Fe1-xVx)3Al ( 0

Dynamical conductivity spectra [σ(ω)] have been measured for many heavy-fermion (HF) Ce and Yb compounds. A characteristic excitation peak has been observed in the infrared region of σ(ω) for all the compounds, and has been analyzed in terms of a simple model based on conduction (c)-f electron hybridized band. A universal scaling is found between the observed peak energies and the estimated c-f hybridization strengths of these HF compounds. This scaling demonstrates that the model of c-f hybridized band can generally and quantitatively describe the low-energy charge excitations in a wide range of HF compounds.

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The optical conductivity [ð!Þ] of the Kondo semiconductor YbB 12 has been measured over wide ranges of temperature (T ¼ 8{690 K) and photon energy (h ! ! 1:3 meV). The ð!Þ data reveal the entire crossover of YbB 12 from a metallic electronic structure at high T's to a semiconducting one at low T's. Associated with the gap development in ð!Þ, a clear onset is newly found at h ! ¼ 15 meV for T 20 K. The onset energy is identified as the gap magnitude of YbB 12 appearing in ð!Þ. This gap in ð!Þ is interpreted as the indirect gap, which has been predicted by the renormalized-band model of the Kondo semiconductor. On the other hand, the strong mid-infrared (mIR) peak observed in ð!Þ is interpreted as arising from the direct gap. The absorption coefficient around the onset and the mIR peak indeed show the characteristic energy dependences expected for indirect and direct optical transitions in conventional semiconductors.

Optical conductivity [sigma(omega)] of YbS has been measured under pressure up to 20 GPa. Below 8 GPa, sigma(omega) is low since YbS is an insulator with an energy gap between a fully occupied 4f state and an unoccupied conduction (c) band. Above 8 GPa, however, sigma(omega) increases dramatically, developing a Drude component due to heavy carriers and characteristic infrared peaks. It is shown that increasing pressure has caused an energy overlap and hybridization between the c band and 4f state, thus driving the initially ionic and insulating YbS into a correlated metal with heavy carriers.

Abstract. Electronic excitations in transition-metal oxides MnO, FeO, CoO and NiO are investigated by inelastic X-ray scattering and optical reflectivity measurements. The dielectric functions are derived from the experimental data as a function of the momentum transfer, q. Based on the derived q-dependent dielectric functions, two types of the charge transfer excitations, i.e., dipolar and non-dipolar charge transfer, are clearly identified. We show that the Mott gaps around 5 eV are defined by the former whereas the latter occurs at higher energies of 8-12 eV. Based on a molecular orbital analysis, we associate the dipolar and the non-dipolar excitations with non-local charge transfer and conventional charge transfer, respectively. These types of excitations are shown to be common for the 3d metal monoxides. On the other hand, the dd excitations observed in NiO and CoO at energy <4 eV do not appear in FeO and MnO. The reasons are addressed in this report.PACS. 71.20.-b Electron density of states and band structure of crystalline solids -78.70.Ck X-ray scattering